1. Field of the Invention
The present invention relates to a solid-state image sensor and an image pickup apparatus including the same. More particularly, the present invention relates to a solid-state image sensor capable of enhancing image quality by stabilizing a signal level, e.g. a black level, and capable of accurately correcting the black level when mounted on a digital camera, scanner or similar image input apparatus, and to an image pickup apparatus including the same.
2. Description of the Background Art
A digital camera, for example, is configured to pickup a desired subject by focusing the optical image of the subject on a solid-state image sensor. The current trend in the imaging art is toward a solid-state image sensor having pixels densely arranged thereon for improving the quality of an image picked up thereby. Japanese patent laid-open publication No. 2002-290841, for example, discloses an image pickup apparatus including an effective pixel area, or image pickup area, and a plurality of optical black (OB) areas. The image pickup apparatus disclosed is constructed such that, when intense light is incident on one optical black area, an accurate reference black-level signal is obtained from the remaining optical black areas in order to reduce blackout.
Japanese patent No. 2806035, for example, teaches a solid-state image sensor including an effective pixel area and an optical black area surrounding it. In this case, when the image of a subject picked up by the image sensor is to be reproduced from image data representative of the image, a black level is corrected by use of data output from the optical black area. The above patent further teaches a video signal processing circuit for controlling the potential variation of a video signal during black-level periods.
However, the prior art schemes stated above have a problem that a plurality of optical black areas must be arranged above and below the effective pixel area in the imaging plane, and a problem that control over switching of the optical black areas and other sophisticated control processing are required. Consequently, it is difficult to further enhance image quality in a high-sensitivity pickup mode, a long-exposure pickup-mode or similar image pickup mode.
A conventional optical black area is implemented by a shield layer formed on the top of the individual photoelectric transducer or photosensitive cell. This, however, gives rise to a problem that, when the level of an output signal from the optical black area varies due to temperature elevation or extremely long exposure time and rises relative to the black-level output of the effective pixel area, the variation of a black signal output from the effective pickup area is cut off by the black level of the optical black area, rendering dark regions practically “flat black”. Once the variation of black gradation picked up by the effective pixel area is cut off by the output level of the optical black area, then the variation of the black gradation cannot be restored. It is therefore necessary to correct the black level with high accuracy.
On the other hand, when temperature is low or when exposure time is extremely short, the black level output from the effective pixel area rises relative to the black level of the optical black area, rendering dark regions light. This problem, however, can be coped with by signal processing and therefore not so critical.